Parkinson?s disease (PD) is the second most common neurodegenerative disorder. While the causes of PD are not fully known, the rare familial forms of the disease continue to provide valuable insight into pathways that contribute to disease. For example, alpha-synuclein was discovered to be the major protein component of Lewy bodies only after the gene encoding it was linked to familial PD. Autosomal dominant mutations in LRRK2 are the most frequent genetic cause of PD, with patients most commonly presenting with late onset and classic Lewy body/synuclein pathology. We recently reported that endogenous expression of G2019S LRRK2, the most frequent pathogenic mutation, induces lysosomal abnormalities and the accumulation of insoluble synuclein in neurons. Notably, these effects were reversed by LRRK2 kinase inhibitors. However, the LRRK2 substrates that are aberrantly phosphorylated by G2019S LRRK2 to produce these defects, and whether these changes represent a final common pathologic pathway for all LRRK2 mutations and idiopathic PD, is not known. A subset of the Rab family of small GTPases are now recognized as physiological substrates of LRRK2 kinase activity. Rabs are critically involved in intracellular trafficking and exert powerful influence on the function of many organelles, including lysosomes. We have focused our attention here on Rab8a as a critical intermediary in PD pathogenesis. Rab8a is not only a substrate of LRRK2, but has been linked to other familial and idiopathic forms of PD. While all PD mutations appear to increase Rab phosphorylation, the R1441C mutation in LRRK2 possesses far greater Rab-kinase activity than G2019S. This could translate to greater neuronal dysfunction but has not be adequately addressed. Given the PD-relevant outcomes we recently established for G2019S LRRK2, we will determine here whether the R1441C mutation exerts greater defects in lysosome function and synuclein metabolism using both mouse primary neurons and human iPSC-derived neurons. We will then examine the relative effects of G2019S and R1441C LRRK2 on Rab8a biochemistry and biology, and dissect the precise role of Rab8a in LRRK2-mediated neuronal pathology. Lastly, we will investigate whether dysregulation of LRRK2 signaling pathways contribute to idiopathic PD. Through a careful analysis of post mortem brain tissue from idiopathic PD patients and age-matched controls, we will interrogate the status of multiple upstream and downstream features of LRRK2 protein and markers of LRRK2 kinase activity. We have assembled a team with deep expertise in cell biology and neuropathology and will complete a series of critical and timely experiments to provide much-needed insight into the mechanistic and pathogenic consequences of disease-linked LRRK2 mutations in neurons, and whether these pathways are involved in the far more common idiopathic cases of PD.